This invention relates to a method for conditioning, by acid swelling, crude perylene pigment using low concentrations of toluene sulfonic acid at a particular range of temperature, and more particularly to an improved process of finishing perylene pigment in a finely-divided form exhibiting high tinctorial strength and tinctorial stability in lacquers and in coloration of plastics.
It is conventional to condition crude organic pigment such as phthalocyanine, quinacridone, perylene, carbazole violet by a method known as acid swelling. Crude pigment is generally unsuitable for use as pigment and typically must undergo finishing treatments to modify its particle size and crystal structure so as to make it pigmentary grade. Such a process involves mixing crude pigment in sulfuric acid of certain concentration. With such concentrations of acid, the crude pigment does not dissolve nor is it crystallize but is caused to swell and after removal of the liquid and drying, the pigment becomes a soft textured product having a very fine particle size. The amount of acid used in this treatment must be sufficient to obtain a stirrable slurry. Generally, a ratio on the order of one part by weight of crude pigment to 10 to 20 parts by weight of acid has been found to produce satisfactory results.
In all cases, the method of acid swelling requires significant amounts of acid which unavoidably increases production cost in order providing an efficient agitation and effective transfer from the acid swelling reactor, for example, to a digestive solvent or water for further treatment.
The present invention provides a method of conditioning crude perylene pigment comprising treating 1 part by weight of the crude pigment with about 2 to 10 parts by weight of toluene sulfonic acid at temperatures ranging from about 40 to 140xc2x0 C., thereby generating an acid swelled perylene pigment.
Other objects and advantages of the invention will become apparent from the following description and appended claims.
It has now surprisingly been found that the conditioning of crude perylene pigment can be achieved in a high energy mixer through acid swelling the crude in about 2 to 10 parts, by weight, preferably about 3 to 5 parts by weight, of toluene sulfonic acid at temperatures ranging from about 40 to about 140xc2x0 C. The resulting pigment-phosphate salt can then be digested in water or in a water miscible organic solvent, to provide a finely-divided product exhibiting high tinctorial strength and excellent dispersibility in lacquers and in coloration of plastics.
Suitable toluene sulfonic acid for the process of the invention include toluene sulfonic acid monohydrate, toluene sulfonic acid anhydride.
Toluene sulfonic acid is added and heated in a glass-lined reactor and crude pigment is then charged to obtain a stirrable mixture. The amount of toluene sulfonic acid is approximately 2-10 fold of crude, preferably 3 to 5 fold by weight.
The amount of acid added will determine the physical appearance of the resulting mixture which must remain fluid enough to provide a stirrable medium. Higher amounts of toluene sulfonic acid do not harm product quality but adds significantly to production cost. The toluene sulfonic acid can be mixed with crude through a blender to form a uniform mixture and the resulting mass is allowed to set in an oven. This procedure provides equally good material.
The toluene sulfonic acid mixture is generally heated to a temperature of from about 40 to 140xc2x0 C., more particularly 50 to 120xc2x0 C., for about 2 to 12 hours, and more preferably at 6 to 10 hours. The time for heating will depend on the solvent selection and pigment. The crude pigment reacts with toluene sulfonic acid to form a very dense or less dense mixture depending on toluene sulfonic acid and crude ratio. Higher ratios of toluene sulfonic acid to crude provides better tinctorial strength. Lower swelling temperature also results in improved properties in both paints and plastics.
For further conditioning, the entire acid swelled medium is digested with cold water either through direct strike (i.e. adding the acid medium slowly into ice water) or by an indirect strike, (i.e. adding water into the acid medium). The amount of water used is preferably about 4 to 8 times by weight of the swelled pigment in the medium. The resulting water slurry is then filtered, washed with water until low conductivity is reached and the resulting presscake is reslurried into a water miscible organic reslurry solvent or water and heated at greater than atmospheric pressure, depending on the solvent selected, to a temperature of from 80 to 160xc2x0 C., preferably 120-160xc2x0 C. for about 2 to 6 hours.
Suitable reslurry solvents include lower alcohols, ketones, acids or other basic solvents, for example, methanol, ethanol, n-propabol, isopropanol, n-butanol, iso-butanol, pentanol, hexanol, acetone, methyl ethylketone, methylisobutylketone, glacial acetic acid, dimethylforamide, dimethylsulfoxide, N-methylpyrolidone, or mixtures thereof.